We are continuing our effort to evaluate the physiological role of our purified serum-suppressor factor, which we have termed "suppressive E-receptor (SER)" factor. SER acts to inhibit the proliferation of growth-factor-dependent normal B or T cells in culture, i.e., BCGF-dependent human B-cell line BD-7 and TCGF-dependent murine T-cell line, CT-6. With the same manner, it also inhibited the production of IL-1 and IL-2 from normal human macrophages and T lymphocytes upon stimulation with lipopolysaccharide and phytohemagglutinin, respectively. However, SER did not interfere with the activities of IL-1 or IL-2 once they were produced. This inhibitory effect of SER on cell proliferation can be demonstrated by its direct inhibition on DNA-polymerase of phytohemagglutinin-stimulated, normal human T lymphocytes. Consequently, the inhibitory effect of SER on IL-1 and IL-2 production can be reversed by exogenous addition of IL-1 or IL-2. We are producing monoclonal antibody to the SER factor in preparation for use for diagnostic or therapeutic purposes. (HF)